Ionization smoke detector

ABSTRACT

An ionization smoke detector having at least one ionization chamber open to ambient air and connected to a voltage supply, a field effect transistor having a source-drain path connected to said supply and a gate interconnected with said chamber to sense the voltage across the chamber, the source-drain path having a plurality of series connected diodes in series therewith and terminals connected to the junctions between the diodes in order to obtain a desired output voltage from the transistor upon the detection of smoke.

United States Patent [191 Sasaki Feb. 12, 1974 IONIZATION SMOKE DETECTOR [75] Inventor: Koju Sasaki, Tokyo, Japan [73] Assignee: Nittan Company, Limited, Tokyo,

Japan [22] Filed: Oct. 29, 1971 [2]] App]. No.: 193,901

[30] Foreign Application Priority Data Nov. 19, 1970 Japan 45-114352 [52] US. Cl 250/83.6 R, 307/251, 340/237 S [51] Int. Cl. G0lt 1/18 [58] Field of Search 340/237 S, 237 R;

[56] References Cited UNITED STATES PATENTS 3,559,196 1/1971 Scheidweiler 250/836 3,435,257 3/1969 Lawrie 307/291 3,089,968 5/1963 Dunn.... 307/230 3,395,265 7/1968 Weir 219/209 11/1970 Gilbert 307/235 OTHER PUBLICATIONS The What & Why of integrated Circuits" by Stern, Radio-Electronics, October 1963, pages 34, 35.

Primary Examiner-James W. Lawrence Assistant ExaminerHarold A. Dixon Attorney, Agent, or Firm-Eugene E. Geoffrey, Jr.

[57] ABSTRACT 1 Claim, 2 Drawing Figures PATENTEI] FEB I 21974 Or/Z FIG. I PRIOR ART INVENTOR. K6U86145Anf/ 1 IONIZATION SMOKE DETECTOR This invention relates to an ionization smoke detector and more particularly to an ionization smoke detector provided with a sensitivity control.

An ionization smoke detector of the type concerned includes a closed ionization chamber and an open ionization chamber each having a pair of electrodes and a radioactive source and connected in series between a pair of terminals to be connected to a central unit, a field effect transistor (hereinafter referred to as FET) having a gate electrode connected to the connection point of both ionization chambers and a source-to-drain path connected between both terminals. A relay element such as silicon controlled rectifier (hereinafter referred to as SCR) has a control electrode connected to the source electrode of the FET and a main conduction path connected between both terminals. A specific operation voltage is applied between both terminals from the central unit in the normal monitoring condition. When smoke enters the open ionization chamber, the ionization current flowing in the open ionization chamber is reduced and thereby the impedance of the open ionization chamber is increased to raise the potential at the junction point of both ionization chambers and, accordingly, the gate potential of the F ET. This results in an increase of the source voltage, and when the source voltage exceeds a specific value, the SCR is driven into conduction and shortcircuits both terminals to transmit a detection signal to the central unit.

It is desirable to have an ionization smoke detector whose sensitivity is controllable, since it is sometimes necessary to change the sensitivityof each ionization smoke detector to meet certain operating conditions. For this purpose it has been suggested that a potentiometer be inserted in series with the source-to-drain path of the FET to control the voltage applied to the control electrode of the SCR. However, the potentiometer is costly and delicate and, therefore, unsuitable for use as a structural element of such device. It has also been proposed to substitute a series connection of fixed resistors having tapsfor the potentiometer, but this method has the disadvantage that the tap voltages of the series resistors vary significantly with the current flowing therethrough and the sensitivity is therefore not stabilized.

Accordingly, an object of this invention is to propose an ionization smoke detector having aninexpensive and strongly built sensitivity control which affords improved stability.

According to this invention, this object is attained by inserting a series connection of a plurality of diodes in series with the source-to-drain path of the FET.

Now, the invention will be described moreclearly hereinunder with reference'to the accompanying drawings.

FIG. 1 is a schematic circuit diagram representing an ionization smoke detector including a sensitivity controlaccording to prior art, and

FIG. 2 is a schematic circuit diagram representing an ionization smoke detector including a sensitivity control according to this invention.

Throughout the drawings like reference numerals are used to denote corresponding elements.

Referring to FIG. 1 representing a prior art device, the ionization smoke detector includes a closed ionization chamber 10 having a pair of electrodes 1 l and 12 and a radio-active source 13 and an open ionization chamber 20 having a pair of electrodes 21 and 22 and a radioactive source 23. Both the ionization chambers 10 and 20 are connected in series between a pair of terminals l and 2 which are to be connected to a central unit (not shown). The connection point 3 of both ionization chambers is connected to the gate electrode G of an FET 4 having a drain electrode D connected to the terminal 1 and a source electrode S connected through a resistive'element of a potentiometer 8 and a load resistor 5 to the terminal 2. The movable contact of the potentiometer 8 is connected to the control electrode of an SCR 6 whose main conduction path is connected between both terminals 1 and 2. The control electrode of the SCR 6 is also connected through a leakage resistor 7 to terminal 2.

When smoke enters the open ionization chamber 20 as described above, the potential at the connection point 3, and accordingly the gate electrode G of the FET 4 is raised to increase the drain-to-source current, thereby increasing the source voltage and, accordingly, the voltage at the control electrode of the SCR 6. When this voltage exceeds a specific threshold value, the SCR is driven into conduction to short-circuit both' terminals 1 and 2. i

The sensitivity of the detector can be controlled by moving the movable contact'9 of the potentiometer 8 to control the voltage to be applied to the control electrode of the SCR 6. However, such mechanical contact 9 is often defective or makes improper contact and, thus, reduces the reliability of the device.

Referring next to FIG. 2 representingan embodiment of this invention, the circuit is the same as that of FIG. 1 except that a series connection of a plurality of diodes 31, 32, 33, is inserted in place of the potentiometer 8 of FIG. 1. Terminals 41, 42, are respectively led out from the connection points between the respective diodes and a contact 9 which is connected to the control electrode of the SCR 6 is selectively connected to one of the terminals 41, 42, The contact 9 may be constructed as a movable change-over contact, but it is preferable to fixedly connect it to one terminal which is previously selected in accordance with its situation, by means of screwing or soldering, in order to prevent improper'contact.

Each of the diodes exhibits a voltage drop of about 0.5 volts, for example, and this value is maintained almost constant in spite of variations in the current flowing therethrough. Therefore, the terminals 41, 42

. provide voltages which are lower than the source voltage of the FET 4;by precisely 0.5 volts and its integral multiples, respectively, regardless of the drain-tosource current. Therefore, once the terminal of the diode train is selected to set the sensitivity of the deodes may be adopted as the diodes 31, 32, 33, so as to further improve the accuracy and reliability of actuation of the device.

What is claimed is:

1. An ionization smoke detector comprising at least one open ionization chamber including a pair of electrodes and a radioactive source, means for applying a voltage to said electrodes, a field effect transister having gate, drain and source electrodes, a connection between one of said chamber electrodes and said gate electrode, a plurality of series connected diodes fortrance of smoke into said chamber. 

1. An ionization smoke detector comprising at least one open ionization chamber including a pair of electrodes and a radioactive source, means for applying a voltage to said electrodes, a field effect transister having gate, drain and source electrodes, a connection between one of said chamber electrodes and said gate electrode, a plurality of series connected diodes forwardly inserted in series with said source electrode, means for applying a voltage to said source and drain electrodes through said series connected diodes, a connection between the other of said chamber electrodes and said series connected diodes, an output terminal and means for connecting said output terminal to selected junctions between said series connected diodes to vary the magnitude of said output signal upon the entrance of smoke into said chamber. 